Concrete is a mixture of sand and/or gravel and Portland cement and, when mixed with water, forms a slurry that can be molded into virtually any shape. After such mixing, a chemical reaction occurs in the presence of water that causes the slurry to harden over the course of several days (often referred to as curing or, much less accurately, as drying since the concrete must remain hydrated for curing to take place) into an extremely hard, strong and durable material that is highly weather-resistant and thus especially well suited to large structures such as buildings, roads and bridges. Although concrete is much stronger in compression than in tension, reinforcing materials such as iron rods can be embedded in the structure as it is cast in-situ or formed into prefabricated components for later assembly in order to develop overall strength in tension.
However, concrete has several properties that are not optimal for some applications for which it is otherwise well-suited. Specifically, it is very heavy, having a weight per unit volume comparable to stone and, while weight can be desirable in some applications, such as dams, anchors and the like, structures having a large proportion of concrete need very substantial support. Also, Portland cement is very expensive and filler materials such as sand or gravel are usually added to the mixture to a proportion as large as possible without weakening the final cured concrete product beyond specifications. Further, while concrete has a high specific heat and is capable of storing substantial energy in the form of heat, it is also highly thermally conductive (again, substantially comparable to stone) and thus generally requires insulation if used for the perimeter of buildings. Further, the combination of hardness and weakness in tension causes substantial difficulty in further working (e.g. cutting, drilling, setting nails or screws and the like) of the concrete shapes once they have fully cured. For example, nails to be driven into fully cured concrete must generally be hardened and resistant to bending and require very high force to be applied to drive them into concrete while nails that have been successfully driven may be found to be loose and are not solidly retained by the concrete while the force required to drive them may crack or at least weaken the concrete or cause persistent stresses that may do so over time.
To alter these properties, there has been much interest in concrete compositions having a relatively high content of wood, cellulose or paper fiber therein, especially as a technique for recycling of discarded paper such as newsprint which is generated in large volume. However, while there has been some success in developing such concrete-based compositions, sometimes referred to as papercrete, the processing of paper to obtain a proper consistency by techniques developed to date has proven to be energy-intensive, time-consuming and expensive. Further, mixing of such compositions is difficult (possibly due to the differences in buoyancy and water absorption of wood/paper fiber and other constituent materials) and has generally been done in small batches of a fraction of a cubic yard in a process that is not easily scalable to larger quantities consistent with delivering repeatably acceptable and substantially uniform results.
Another difficulty presented by the use of concrete in construction is the need to build large and strong forms of other materials such as wood or metal when concrete is to be cast in-situ or to form construction modules which can represent a significant cost of the finished structure. When similar shapes are to be formed, some expense can be avoided by re-use of such forms. However, such re-use for in-situ concrete construction is labor intensive and cost savings are marginal but may be economical in forming a large number of similar prefabricated shapes that can later be assembled into a structure.
To avoid some of the labor costs for forming concrete shapes which are largely vertical, however, a technique called slip-forming has been developed which involves multiple pours of concrete mix as the form is incrementally moved. However, this technique has proven somewhat dangerous since the degree of curing of a given pour of concrete mix must be sufficient to support the weight of both the next and further subsequent concrete pours as well as the form into which such pours are made and machinery to compact the concrete mix within the form. Numerous construction accidents have occurred when a sufficient cure of the concrete mix is not achieved prior to a subsequent pour. Therefore, slip form techniques are inherently slow when performed safely. Conversely, if the cure is more complete than necessary for adequate structural support, one pour may not adhere to or integrate sufficiently with a previous pour, leaving regions of weakness and/or persistent stress within the completed concrete shape.